Nervous and Endocrine Systems: How the Body Coordinates Itself

Touch something hot and your hand pulls back before you decide to move it. Digest a meal and, minutes later, your blood sugar is held steady. Stay awake for two days and you feel alert but unsettled. These are not separate events. All of them are the same two systems at work.

The nervous system carries information as electrical impulses along fibres. The endocrine system carries it as chemicals called hormones in the blood. They differ in speed, range and duration, and the body needs both.

Two Systems, Two Speeds

The nervous system is the faster of the two. A signal can travel from the fingertip to the spinal cord and back in a fraction of a second. Its messages are also aimed carefully. A nerve fibre runs to one place, so a command to move a calf muscle travels down the fibre that serves that muscle.

The endocrine system is slower and far broader. A hormone released into the blood reaches much of the body, so it cannot be aimed at one spot. What makes it precise instead is selectivity: only cells with a matching receptor respond. The trade-off is duration. A nerve impulse is over in milliseconds, while a hormone such as cortisol keeps working for hours.

	Nervous system	Endocrine system
Signal	Electrical impulses, then neurotransmitters	Hormones in the blood
Speed	Milliseconds	Seconds to hours
Route	Wires — axons carry a signal to one target	Broadcast — blood carries it everywhere
Targeting	Precise: one fibre, one destination	Selective: only cells with a matching receptor
Duration	Brief	Long-lasting
Typical jobs	Reflexes, movement, sensation, thinking	Growth, metabolism, sleep, reproduction, stress

Neither system is more important. A reflex with no hormonal support would still jerk your hand back. But you could not grow, sleep, repair or reproduce on electrical signals alone.

The Nervous System: Wires, Switches and Signals

Nervous tissue first arose in worm-like animals about 550 to 600 million years ago. Its working unit is the neuron, a cell that fires electrical signals called action potentials. Neurons form networks called neural circuits. They pass information to one another across synapses, which are junctions that usually hand the signal over with a chemical neurotransmitter.

The Grand Division: Central and Peripheral

In vertebrates the nervous system splits in two. The central nervous system is the brain and spinal cord: the processing center. The peripheral nervous system is the bundle of nerves that connects that center to the rest of the body. Nerves carrying information toward the CNS are sensory (afferent). Nerves carrying commands away are motor (efferent).

The peripheral system divides again. The somatic nervous system controls voluntary movement. The autonomic nervous system controls everything you do without thinking, and it has three parts: sympathetic, parasympathetic and enteric. The sympathetic branch releases energy in emergencies. The parasympathetic branch handles feeding, breeding, rest and digestion. The enteric system runs the gut, and it is sometimes called a second brain. It counts either as part of the autonomic system or as a separate one.

The peripheral nervous system splits into the voluntary somatic system and the unconscious autonomic system. Image: Christinelmiller, CC BY-SA 4.0, via Wikimedia Commons

The Neuron and Its Signal

An action potential is a burst of electrical activity. The voltage across a cell membrane rises fast, then falls. That rise is called depolarization, and it triggers the same thing in the membrane next door. So the signal travels down the axon like a wave. Myelin insulation around many axons lets the signal jump between gaps, in a process called saltatory conduction. That is far faster than creeping along bare membrane. When the wave reaches the end of the axon, the neuron releases neurotransmitter across the gap onto the next cell.

Synapses come in two kinds. Chemical synapses are one-way and flexible, which is why learning can strengthen or weaken them. Electrical synapses join the insides of two cells through gap junctions, so signals pass both ways at once. That is fast and reliable, which suits groups of cells that must fire together.

A neuron receives signals along its dendrites, fires from the cell body, and passes the signal on down the axon to other cells. Image: Amyleesterling, CC BY-SA 4.0, via Wikimedia Commons

A multipolar neuron, the most common shape among neurons, has many dendrites feeding signals into one axon. Image: BruceBlaus, CC BY 3.0, via Wikimedia Commons

Three Jobs Neurons Do

Neurons are usually sorted into three groups by function. Sensory neurons respond to touch, sound or light and send signals to the spinal cord and on to the brain. Motor neurons receive commands from the brain and spinal cord and carry them out, controlling everything from muscle contraction to glandular output. Interneurons connect neurons to other neurons in the same region. They do the processing behind memory, learning and decision-making.

The Reflex Arc: Thinking Afterwards

A reflex arc is a shortcut that bypasses the thinking part of the brain. In a somatic reflex, most sensory neurons synapse in the spinal cord, and the cord activates motor neurons directly. The brain still gets the information, but it arrives while the movement is already underway. The analysis happens afterwards. That is why your hand is off the hot pan before you feel it.

The pathway runs in five steps:

• Receptors in the skin, muscles or tendons detect the stimulus.
• Afferent fibres carry the signal to the posterior horn of the spinal cord or to the brainstem.
• An integrating center in the grey matter forms the synapse.
• Efferent fibres carry motor signals out from the anterior horn to the muscles.
• The effector muscle performs the response.

Reflex arcs come in two kinds. Autonomic reflex arcs act on internal organs, and somatic ones act on muscles. The division is not absolute. Some autonomic reflexes involve the spinal cord, and some somatic reflexes are handled more by the brain than the cord.

The Endocrine System: Chemical Broadcast

The endocrine system works through glands that release hormones straight into the blood. A hormone circulates until it meets a cell with a matching receptor. One release therefore reaches every cell of that type in the body at once. That is how the pituitary can direct the thyroid, the adrenal cortex and the reproductive glands all at the same time.

Three glands dominate everyday function. The pituitary, pea-sized and suspended below the hypothalamus, is called the master gland because its hormones command the others. The thyroid sets the pace of metabolism through thyroxine and triiodothyronine. The adrenal glands, one on top of each kidney, release cortisol and aldosterone from their outer layer and adrenaline from their inner one.

The Feedback Axis

Hormone levels are not simply switched on and off. A loop running from the brain down to a target gland and back holds them in range. The hypothalamus releases a releasing hormone, which tells the pituitary to secrete a tropic hormone. The tropic hormone travels in the blood to a target gland, the thyroid say, and makes it release its own hormone. As that hormone rises in the blood, it feeds back to suppress the hypothalamus and the pituitary. This negative feedback is what stops the system from overshooting.

The hypothalamus directs the pituitary, which in turn directs the other endocrine glands. Image: OpenStax College, CC BY 3.0, via Wikimedia Commons

The same loop runs in reverse for stress. When cortisol falls, the pituitary releases ACTH, so the adrenal cortex makes more cortisol. Rising cortisol then switches the pituitary back off. Both halves of that loop can be measured, which is why hormone tests tell doctors so much.

Where the Two Systems Meet

The hypothalamus is the joining point. In humans it is about the size of an almond, and it sits below the thalamus as part of the limbic system. It has one job more important than any other: it links the nervous system to the endocrine system through the pituitary gland. It also regulates body temperature, hunger, thirst, fatigue, sleep and circadian rhythms, and it controls parts of parenting behavior and some social behaviors. It is a piece of brain tissue doing a gland’s job.

The Stress Response: One System Triggering the Other

The clearest proof that these systems are one system is what happens when you are frightened. The fight-or-flight response was first described by Walter Bradford Cannon in 1914 and 1915. He argued that animals meet a threat with a general discharge of the sympathetic nervous system, preparing them to fight or flee.

Here is the sequence, and note where the two systems hand over to each other:

• A threat is detected. Sensory neurons report it to the brain, and the sympathetic nervous system activates.
• Sympathetic preganglionic neurons release acetylcholine onto cells in the adrenal glands.
• Those cells — the chromaffin cells of the adrenal medulla — release catecholamines: epinephrine, norepinephrine and a little dopamine.
• Because these hormones enter the blood, the effect is no longer local. Heart rate, blood pressure, blood flow to the muscles and glucose availability all rise, body-wide, for as long as the signal lasts.
• Cortisol follows on a slower track, raising blood sugar and damping inflammation so the response can be sustained.

Sympathetic nerve fibres supply the adrenal medulla directly, turning a nerve signal into a hormonal one. Image: Blausen.com staff, CC BY 3.0, via Wikimedia Commons

Note the elegance of the arrangement. A nerve impulse reaches a single adrenal gland within milliseconds. The gland then turns that electrical message into a chemical one that floods the whole body. The nervous system supplies the precision, and the endocrine system supplies the reach. Neither could do the other job.

Stress hormones also follow a daily rhythm. They are not released only in crises. Cortisol rises in the early morning, falls through the day, and reaches its lowest point late at night. That pattern is set by a circadian rhythm, which is one more reminder that the two systems never work separately.

Cortisol is released on a daily cycle controlled by the body clock, not only under stress. Image: Obry, CC BY-SA 4.0, via Wikimedia Commons

What Both Systems Are For

The purpose of all of this is homeostasis: keeping internal conditions inside a range the body can live in while the outside world changes. Temperature, blood glucose, blood pressure, fluid balance, blood pH, oxygen and carbon dioxide levels, and the levels of sodium, potassium and calcium are all held this way.

The two systems divide the work by timescale. A sudden change gets a nervous response, because a nerve impulse arrives in under a second. A change that lasts gets a hormonal response, because only a hormone can hold an effect for hours. Run a marathon and your nerves tell your muscles to keep contracting while your heart rate and breathing respond at once. At the same time, your endocrine system releases adrenaline, cortisol and insulin to supply and manage fuel. You are not choosing between the two systems. Both are always running.

Key Terms in Plain English

These are the words in this article that would send you to a dictionary. Each one is given here the way it is actually used above.

Term	What it means here
Neuron	A nerve cell that fires electrical signals and passes them on.
Action potential	The fast rise and fall of voltage across a nerve cell membrane.
Synapse	The junction where one neuron passes a signal to the next cell.
Neurotransmitter	The chemical that carries a signal across a synapse.
Reflex arc	The short route that lets the spinal cord act before the brain has finished deciding.
Hormone	A chemical messenger released into the blood to act on distant cells.
Gland	A tissue that makes and releases a substance such as a hormone.
Pituitary	The small gland below the hypothalamus that directs the other endocrine glands.
Hypothalamus	The brain region that links the nervous and endocrine systems.
Negative feedback	The loop in which a rising level switches off the signal that made it.
Homeostasis	Keeping internal conditions inside the range the body can live in.

The words in this article that would otherwise send you to a dictionary.

Key Takeaways

• The nervous system sends electrical impulses at millisecond speed to exact targets. The endocrine system broadcasts hormones through the blood, more slowly but for far longer.
• Hormone targeting works by receptor specificity, not by aiming. Only cells with a matching receptor respond.
• The peripheral nervous system divides into the somatic system, which is voluntary, and the autonomic system, which runs the sympathetic, parasympathetic and enteric branches.
• Reflex arcs let the spinal cord respond before the brain has finished processing, which is why a hand pulls back from heat instantly.
• The hypothalamus links the two systems and controls temperature, hunger, thirst, sleep and circadian rhythms through the pituitary.
• The stress response proves they are one system: sympathetic nerves trigger the adrenal medulla, which releases adrenaline throughout the body.
• Both systems exist to hold homeostasis, dividing the work by timescale: nerves for seconds, hormones for hours.

Frequently Asked Questions

What is the main difference between the two systems?

Speed, and how the message travels. Nerve impulses are electrical, run along dedicated fibres, reach one exact destination and are over in milliseconds. Hormones are chemical, travel in the blood, reach the whole body, but affect only cells with a matching receptor, and keep working for hours. One is a phone call. The other is a broadcast that only some receivers are tuned to.

Which system is more important?

Asking which matters more assumes one can be removed, and it cannot. The nervous system without hormones gives you reflexes and thought but no growth, no control of metabolism and no reproduction. The endocrine system without nerves gives you steady chemistry but no fast response to danger. In a real body they are one system. The hypothalamus is brain tissue doing a gland’s job, and sympathetic nerves are what start the hormonal stress response.

How is adrenaline involved in both systems?

Adrenaline is made by the chromaffin cells of the adrenal medulla, which is an endocrine gland, and it travels in the blood, so it is a hormone. Yet it is released in answer to a nerve signal, because sympathetic preganglionic neurons stimulate those cells directly. Adrenaline is therefore the point where the two systems touch. It turns a fast electrical command into a slow chemical one that reaches the whole body.

Do neurons and hormones ever work on the same target?

Often. Noradrenaline is a good example. It acts as a neurotransmitter in the brain and as a hormone in the blood, from the same molecule and often from the same cells. The difference is not the chemistry but the reach. Released across a synaptic gap, it affects one cell. Released into the blood, it reaches every cell with the right receptors.

Sources: Wikipedia articles on the nervous system, neurons, action potentials, synapses, reflex arcs, the autonomic and sympathetic nervous systems, the endocrine system, the hypothalamus, the adrenal medulla, the fight-or-flight response and homeostasis. Images: Wikimedia Commons, with authors and licenses noted in each caption.